1994 4.6 EFI heads on a 1981 inline SIX?

[Joediveguy]

Hey y'all. When I picked up my 1981 Bronco, AKA, Ugly Betty, it came with an extra inline SIX from a 1994 F150 that I plan on rebuilding. The original SIX runs as is but my plan is to rebuild the '94 SIX at some point and install it later on down the road. My question is, in the mean time, can I swap the '94 heads now on the '81 SIX or is there anything that needs to be changes on them?

The research I have done says the newer EFI heads and the dual exhaust manifolds breath better and I would like to swap them now while the short block is being rebuilt. If it will make it run better.

Any input would be helpful. Thank you.

[1986F150Six]

I believe the latter head will swap, but keep in mind that it has fast burn characteristics and the compression will be approximately 1/2 point higher. It will have 3 additional bolts to facilitate with the dual exhaust manifolds which were offered when F.I. was introduced [1987].

[Rembrant]

My question is, in the mean time, can I swap the '94 heads now on the '81 SIX or is there anything that needs to be changes on them?

The EFI head will have six holes that will have to be plugged on the exhaust side. They are for the Thermactor system...see the pipe running the length of the head below? There was a guy on here swapping one of these heads on recently. Not sure what he used to plug the holes.

PS: On swapping the whole engine in later, keep in mind the later EFI blocks did not have a hold to mount the mechanical fuel pump. David might know more...I believe the hole can be cut out and the bolt holes tapped, but as it stands they're all blank.

 

[1986F150Six]

My question is, in the mean time, can I swap the '94 heads now on the '81 SIX or is there anything that needs to be changes on them?

The EFI head will have six holes that will have to be plugged on the exhaust side. They are for the Thermactor system...see the pipe running the length of the head below? There was a guy on here swapping one of these heads on recently. Not sure what he used to plug the holes.

PS: On swapping the whole engine in later, keep in mind the later EFI blocks did not have a hold to mount the mechanical fuel pump. David might know more...I believe the hole can be cut out and the bolt holes tapped, but as it stands they're all blank.

You are correct regarding the lack of mechanical pump, Cory. Using a fuel pump gasket as a template, the hole can be cut out. Some choose to use an electric fuel pump instead.

Per Bill Vose:

Those may go into the exhaust ports and would be for the smoke grinder (Thermactor system) if they are threaded. If not threaded do as Gary recommends. If your truck does not have that, they can be plugged. They should be a 7/16-20 thread, but could be 3/8-24. Which ever they are get six headless Allen screws (set screws are good due to the point) and put them in tightly.

Bill Profile

[JimJam300]

You should check out the fordsix forums. They got some wizards up in there.

From what I've read, the EFI head doesn't flow as well at low valve lift, combustion chamber is smaller (higher compression), combustion chamber shape is "fast-burn" for fuel economy. They are also prone to cracking. There are new manufactured ones that are more stout and big valve CNC ported ones as well. No such off-the-shelf improvements for carb'd heads that I know of.

You might need to look into what cam to run.

I'm also doing the research for a build/rebuild on my own 300: carb'd head with big valves, porting, cam, aftermarket intake manifold, EFI exhaust manifolds, and Holley Sniper EFI.